(1) Field of the Invention
The present invention generally relates to a base-band delayed detector, and particularly to a base-band delayed detector in which a phase of a clock signal used for a delayed detecting operation is controlled based on an output signal of the delayed detecting operation.
(2) Description of Related Art
A conventional base-band delayed detector operating in accordance with a .pi./4 shifted QPSK (Quadrature Phase Shift Keying) is shown in FIG. 1.
Referring to FIG. 1, this base-band delayed detector has mixers 51 and 52, a local oscillator 53, a shifter 54, low pass filters 55 and 56, analog to digital converters 57 and 58, an operation circuit 59 and clock generator 63. The mixer 51 converts an IF signal supplied from a circuit (not shown) provided prior thereto into a base band signal having a frequency falling within a base band, by using a local signal output from the local oscillator 53. The shifter 54 shifts a phase of the local signal by .pi./2. The mixer 52 converts the IF signal into a base band signal by using the shifted local signal. Each of the low pass filters 55 and 56 damps a sum component of a corresponding base band signal and allows a differential component thereof to pass through it. The operation circuit 59 carries out a delayed detecting operation with respect to I-channel data and Q-channel data which are obtained through the base band signals supplied thereto respectively via the analog to digital converters 57 and 58. The analog to digital converters 57 and 58 and the operation circuit 59 are operated in synchronism with a timing clock signal output from the clock generator 63. The base-band delayed detector also has data discriminators 60 and 61, a parallel to serial converter 62 and a BTR (Bit Timing Recovery) circuit 64. Each of the data discriminators 60 and 61 generates discriminated data from the output data of the operation circuit 59 in synchronism with a clock signal. The parallel to serial converter 62 converts parallel data supplied from each of the discriminators 60 and 61 into serial data. The BTC circuit 64 generates the clock signal used for operation of the discriminators 60 and 61 and the parallel to serial converter 62.
The IF signal is defined as cos(.omega..sub.c .multidot.t+.phi.) and the local signal is defined as cos(.omega..sub.L .multidot.t+.theta.), where .phi. is a modulation signal component and a relationship .omega..sub.c .apprxeq..omega..sub.L stands. In this case, the above base-band delayed detector operates as follows.
The mixer 51 converts the IF signal cos(.omega..sub.c .multidot.t+.phi.) into a base band signal by using the local signal cos(.omega..sub.L .multidot.t+.theta.). The mixer 52 converts the IF signal cos(.omega..sub.c .multidot.t+.phi.) into another base band signal by using the shifted local signal -sin(.omega..sub.L .multidot.t+.theta.). As the sum components of the base band signals are respectively damped by the low pass filters 55 and 56, the differential component cos(.DELTA..omega..multidot.t+.phi.-.theta.) of the first base band signal is output from the low pass filter 55, and the differential component sin(.DELTA..omega..multidot.t+.phi.-.theta.) of the second base band signal is output from the low pass filter 56. .DELTA..omega. is defined as .DELTA..omega.=.omega..sub.c -.omega..sub.L.
In a case where .DELTA..omega.=0, the differential components are respectively represented by cos(.phi.-.theta.) and sin(.phi.-.theta.). In this case, as only .phi. varies with the passage of time in the signal after passing through each of the low pass filters 55 and 56, an eye pattern in which signals are superimposed seems to stop. There is a phase difference .theta. between the IF signal and the local signal, so that the eye pattern seems to stop in either a state (A) shown in FIG. 2A, a state (B) shown in FIG. 2B or an intermediate state between the states (A) and (B). In a case where .DELTA..omega..noteq.0, the eye pattern seems to gradually vary with the passage of time, in an order of states: (A).fwdarw.(B).fwdarw.(A).fwdarw.(B).fwdarw.. . . In the base-band delayed detector, as .omega..sub.c and .omega..sub.L are in synchronism with each other, the eye pattern normally seems to vary.
The analog to digital converters 57 and 58 respectively output the following signals X.sub.k and Y.sub.k in synchronism with the clock signal of the clock generator 63: EQU X.sub.k =cos(.DELTA..omega..multidot.t.sub.k +.phi..sub.k -.theta.) EQU Y.sub.k =sin(.DELTA..omega..multidot.t.sub.k +.phi..sub.k -.theta.)
The operation circuit 59 carries out the following operation with respect to the above signals X.sub.k and Y.sub.k in synchronism with the clock signal of the clock generator 63: ##EQU1## where t.sub.k is a k-th time at which the analog to digital converters 57 and 58 performs sampling, t.sub.k-1 is a time prior to the time t.sub.k by one symbol time, X.sub.k, Y.sub.k and .phi..sub.k are respectively X, Y, and .phi. at the time t.sub.k, and X.sub.k-1, Y.sub.k-1 and .phi..sub.k-1 are respectively X, Y, and .phi. at the time t.sub.k-1. As one symbol time .DELTA.t=t.sub.k -t.sub.k-1 is constant and the difference .DELTA..omega. is approximately equal to zero (.DELTA..omega..apprxeq.0), I.sub.k ' and Q.sub.k ' can be represented by the following formulas. EQU I.sub.k '=cos(.phi..sub.k -.phi..sub.k-1) EQU Q.sub.k '=sin(.phi..sub.k -.phi..sub.k-1)
In a case where data (I, Q) is transmitted by a transmitter in accordance with the following relationship between the data (I, Q) and the amount of phase shift .DELTA..phi.(=.phi..sub.k -.phi..sub.k-1),
______________________________________ [I] [Q] [.DELTA. .phi.] ______________________________________ 1 1 .pi./4 0 1 3 .multidot. .pi./4 0 0 -3 .multidot. .pi./4 1 0 -.pi./4 ______________________________________
the following received data (I.sub.k, Q.sub.k) is obtained.
If I.sub.k '&gt;0, I.sub.k =1.
If I.sub.k '&lt;0, I.sub.k =0.
If Q.sub.k '&gt;0, Q.sub.k =1.
If Q.sub.k '&lt;0, Q.sub.k =0.
In the base-band delayed detector described above, it is necessary to supply the clock signals to the analog to digital converters 57 and 58, the operation circuit 59, the data descriminators 60 and 61 and the parallel to serial converter 62. However, in a circuit provided prior to the operation circuit 59, where the eye pattern varies with the passage of time, it is hard to detect edges of modulated signals and to generate a clock signal having a pertinent phase.
Thus, in the conventional base-band delayed detector, the clock generator 63 must generate a clock signal having a frequency much larger than a symbol rate of the base-band signal. The clock signal, having a frequency which is, for example, 36 times the symbol rate, is used in operations in the analog to digital converters 58 and 57 and the operation circuit 59.
In addition, it is possible to propose a base-band delayed detector in which the clock generator generates a clock signal having a frequency slightly higher than the symbol rate of the base band symbol, and output data from the operation circuit 59 is converted into an analog signal by a digital to analog converter.
However, in a case where a clock signal having a frequency much higher than the symbol rate is used, the amount of electric power used in the base-band delayed detector is increased. In a case where the digital to analog converter is provided in the base-band delayed detector, a circuit in the base-band delayed detector becomes large scale in nature.